A beam penetration-type color CRT (cathode-ray tube) is generally known and is a display device having a faceplate on which an image or alphanumeric characters can be written. One or more phosphor layers on the inner surface of the faceplate can be selected to emit almost any desired wavelength of visible light. If two layers of phosphor are deposited on the faceplate, it is possible to display more than two distinct colors by changing the depth of penetration of the electron beam into the phosphor layers. Because the electron beam emitted by the cathode in the neck of the CRT strikes the phosphor layers at a velocity influenced primarily by the voltage level on the accelerating anode, a change in the voltage level applied to the accelerating anode will correspondingly change the proportion of light emitted by the two phosphor layers. In other words, in a penetration CRT with two layers of different light emitting phosphor up to about four colors can be displayed to a viewer by changing the DC voltage level applied to the accelerating anode positioned near the front of the CRT.
A significant limitation encountered in the use of penetration-type CRT's is related to the length of the reset period between write periods. Because the DC voltage level on the accelerating anode must be changed during the reset period, the length of the reset period is primarily defined by the electrical capacitance associated with the anode. The anode has a relatively large physical size and, as such, inherently has a large capacitance resulting in a significant amount of electrical charge being stored thereon during a write period. Of course, any additional capacitors, particularly large capacitors which are often used in high voltage power supplies, also increase the capacitance in the high voltage circuit and add to the reset period. Because this electrical charge is increased, or decreased, to change the voltage level on the anode, the reset period separating two write periods is related to the charge/discharge rate inherently associated with the total capacitance seen by the high voltage power supply.
Another limitation found in prior art color switches used with beam penetration CRT's is related to the sequencing of the colors to be displayed on the CRT faceplate. Although it is possible to display between three or four distinguishable colors on a two-layer penetration CRT, some high voltage color switches must operate in a particular sequence. In other words, the high voltage color switch provides one preselected voltage level to the anode in successive write periods, that is, the anode voltage is changed from 10 KV to 14 KV, from 14 KV to 18 KV and finally from 18 KV back to 10 KV. During each of these sequential write periods, images or alphanumerics written by the electron beam are displayed only in that color corresponding to the voltage level impressed on the anode. If images or alphanumerics are to be displayed in one color, such as red, during a particular write period, then at the completion of that write period no additional red information can be displayed until the high voltage color switch sequences through its preselected voltage levels to the next write period at which red information can be displayed.
Of particular interest is U.S. Pat. No. 3,906,333 issued Sept. 16, 1975 to M. Kalmanash for LOW COST SWITCHING HIGH VOLTAGE SUPPLY, assigned to the same assignee as the present case, which describes a switching high voltage power supply for use with a beam penetration-type cathode-ray tube. This power supply has the secondary of a high voltage step-up transformer in series with the accelerating anode of the cathode-ray tube. The primary of the transformer is connected to ground through a capacitor for developing a DC voltage level. This voltage across the capacitor is fed to the regulating input of the baseline DC high voltage power supply. The color switching power supply of the present invention is an improvement over that described in this patent.
Another patent of interest is U.S. Pat. No. 4,092,556 issued May 30, 1978 to D. Chambers et al. for SWITCHED HIGH VOLTAGE POWER SUPPLY SYSTEM. This patent describes a high voltage power supply for the rapid switching of high voltage applied to the anode of a beam penetration color cathode-ray tube. The energy for making the rapid transition between voltage levels is stored in two inductors, one for upward transitions and the other for downward transitions. When it is desired to change the voltage applied to the cathode-ray tube, the appropriate one of the storage inductors is coupled through a control switch to the anode causing the voltage applied to the anode to change at a rapid rate. The voltage rises until the desired voltage level corresponding to a desired upward color is reached at which time the switch is turned off and the storage inductor recharged. A tracking high voltage supply maintains the anode at the predetermined voltage level once that level has been reached.